1. Field of the Invention
The present invention relates to production of thin films; more particularly, to such films having a high degree of thickness uniformity; and most particularly, to a method wherein an magnetorheological fluid finishing system programmably removes material from a thin layer, which may have been previously coated to a non-uniform substrate to leave a layer having a very high level of thickness uniformity.
2. Discussion of the Related Art
Use of magnetically-stiffened magnetorheological fluids for abrasive finishing and polishing of substrates is well known. Such fluids, containing magnetically-soft abrasive particles dispersed in a liquid carrier, exhibit magnetically-induced plastic behavior in the presence of a magnetic field. The apparent viscosity of the fluid can be magnetically increased by many orders of magnitude, such that the consistency of the fluid changes from being nearly watery to being a very stiff paste. When such a paste is directed appropriately against a substrate surface to be shaped or polished, for example, an optical element, a very high level of finishing quality, accuracy, and control can be achieved. A typical MRF finishing system is the QED Technologies Q22 MRF System, available from QED Technologies, Rochester, N.Y., USA.
In a typical magnetorheological finishing system, a work surface comprises a vertically-oriented wheel having an axially-wide rim which is undercut symmetrically about a hub. Specially shaped magnetic pole pieces are extended toward opposite sides of the wheel under the undercut rim to provide a magnetic work zone on the surface of the wheel, preferably at about the top-dead-center position. The surface of the wheel is preferably an equatorial section of a sphere.
Mounted adjacent to the work zone is a substrate receiver and work holder for extending a substrate to be finished into the work zone. The finishing system may be programmed to move the work holder in a plurality of modes and speeds of motion to remove more or less material from the workpiece by varying the areal location of work and speed of travel of the workpiece through the work zone, and therefore the exposure time in the work zone. The finishing may be carried out at any desired angular orientation of the work zone on the carrier wheel, e.g., the workpiece may be positioned on a controllable bed, the carrier wheel positioned over the substrate, and a work zone provided at the bottom dead center position on the carrier wheel.
It is known in the art of thin layer fabrication to coat very thin layers of materials onto substrates. Such layers can be very useful in, for instance, the field of micro-electronics fabrication. For example, it is known to coat a thin layer of silicon on a glass surface of a silicon wafer, the glass being an insulator (xe2x80x9csilicon on insulatorxe2x80x9d, or SOI). It is highly desirable that the silicon layer be very uniform in thickness, typically about 100 nm, and not have cracks extending into or through the layer.
A serious problem can arise, however, in producing coatings requiring a very high level of thickness uniformity and surface integrity. Non-planarities in the substrate may not be followed conformably by the coatings but may tend to be filled in or rounded in the free surface of the coatings; thus, an actual coating may undesirably have thicker and thinner areas, depending upon the topography of the substrate to which it is coated. In the prior art, producing coatings of extreme thickness uniformity and surface integrity can require very complicated manufacturing process including chemical mechanical polishing.
Even in some of such applications, it can be necessary to mechanically or chemically-mechanically finish the upper surface of the coating, as by conventional optical polishing, to achieve a desired absolute thickness. However, such grinding, because of the mechanical stresses required, is known to leave residual microscopic stress fractures in the polished surface. For ultra-thin layers, such fractures may actually extend all the way through the layer, compromising the mechanical and electrical properties of the layer.
In use, the criterion for suitability of such a coating is not its absolute level of flatness but rather its absolute thickness, level of thickness uniformity, and level of surface integrity. Thus, what is needed is a means for providing an ultra-thin layer having very high thickness uniformity and high surface integrity, especially when coated on a substrate having surface non-planar excursions which may be as great as, or greater than, the thickness of the coated layer itself.
It is further needed to be able to provide such a layer with minimal residual stress damage to the layer.
It is a principal object of the invention to provide a method for producing thin layers, and especially on non-planar substrates, wherein the layer so produced has a high level of thickness uniformity and freedom from surface cracks.
It is a further object of the invention to provide a method for finishing of thin layers wherein the finishing leaves minimal stress fracturing of the free surface of the thin layer.
Briefly described, in an improved method for producing a thin layer having highly uniform thickness and freedom from surface or sub-surface cracking, a working layer of the material is formed at a thickness greater than the final thickness desired. The layer may be an independent element or may be formed as a coating on a carrier substrate. An areal (XY) determination of working layer thickness is made by a known technique, such as ellipsometry, laser interferometry, or x-ray diffraction. A currently preferred means is an AcuMap II device, available from ADE Technologies, Inc., Newton, Mass., USA. Data representing a map of thicknesses to be removed from the free surface of the working layer are entered into the control system of a magnetorheological finishing apparatus. The independent layer element or the coated substrate element is mounted on a workpiece holder of the apparatus and correctly indexed to the machine. The MRF machine then removes material as instructed by the control system to leave a residual layer having a very high degree of thickness uniformity at a nominal average thickness and having a very high level of surface integrity.
The invention is useful in providing thin films of materials including, but not limited to, ceramics, glass, metals, transition metal oxides, magnetoresistive alloys, aluminum oxide, nitrides, carbides, gallium arsenide, tungsten, silicon, and sapphire.